Charles Darwin's life embraced two great projects. There was the theory which held that we are risen apes, not fallen angels. And there was the almost equally impressive enterprise of a Victorian gentleman's family.

Randal Keynes's poignant book shows how the two were bound together. Its focus is Charles and Emma Darwin's first daughter, Annie, and her death in 1851, when she was just 10. Annie's box is the writing box in which Emma stowed a few childish relics and kept for the rest of her life, and which Keynes, her great-great-grandson, unearthed by chance.

Keynes uses Annie's life and death to paint a rich portrait of Darwin's domestic sphere. He is careful not to claim too much. Although Annie's death affected Darwin deeply, his theory of evolution was already well developed before she fell ill. And while her death seemed particularly senseless, it was no surprise in those days for parents of 10 children to lose a few.

Darwin's suffering was worse, if anything, because he had been so fascinated by his children. He had, as he said himself, "a fine degree of paternal fervour". And as a keen observer of the natural world he fixed his scientific eye on these most interesting of creatures: his children. Many of the features of human development that found their way into his books struck him when he was playing in the nursery.

Keynes alternates details of life with the Darwins at Down House in Kent with comments on the household reading: from Wordsworth and Hume to childcare manuals and storybooks. The drama of the lengthy gestation of Darwin's mature theory is played out against the background of the incessant round of pregnancy, birth and education of the Darwin children, punctuated by visits from small crowds of cousins.

The Darwin of these pages is an almost saintly figure: patient, reasonable, freethinking, only angered by cruelty. His boundless intellectual curiosity was combined with a relentless honesty. He insisted on sharing his scepticism about salvation with his wife, the devoted but devout Emma Wedgewood. As his father had predicted, this made her fearful about his prospects in the afterlife, and the gulf between them deepened as they grew older together. It was never deeper than when they lost the girl whom Darwin described as "the joy of the household".

The Darwin industry produces biographies regularly, but this one has a rare combination of emotional power and historical authority. Annie's death seems to have reinforced Darwin's doubts about religious consolation. As the author of a theory that relieved God of any responsibility for creating new species, Darwin found it hard to believe that he intervened in human life. And as a grieving parent, he found it impossible to see this death as part of any divine plan. It might have an explanation, even a cause, but no reason. Keynes's Darwin, in other words, is a thinker facing up to the realities of the secular world most of us now live in.

· Jon Turney teaches science communication at University College London